Is this a "stable" build of BBM 5.0? I was one of the unfortunate "few" that had to downgrade because of connectivity issues.

I don't know if you could consider it stable or not as it is only a beta. Most that are using it are saying they are having no issues with it. If you downloaded it when it was first leaked, most likely your connectivity issues were because of the BBM outage, where everyone was affected, not just BBM 5.0 users.